The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
For military or commercial purposes, technologies have been developed for measuring a distance to a particular device or determining a position of a particular device by using a measured distance. Particularly, with the development of a wireless communication technology, the spotlight is on a location based service (LBS) for providing a wireless terminal user with various services based on the user's location. In order to provide the location based service, a technology is required to measure the distance to the user and precisely determines the user's location by using the measured distance.
There have been various ranging methods and positioning methods, and a Round Trip Time of Arrival (RT-ToA) method and a Time Difference of Arrival (TDoA) method are notables among them.
Basic operational principle of the RT-ToA method is described below.
(a) When a transceiver A transmits signal A, a transceiver B receives signal A and transmits reply signal B upon completion of receiving signal A.
(b) Transceiver B measures elapsed time Treply from an end time point of the received signal A to an end time point of the transmitted signal B and then transmits the measured value to transceiver A through a separate signal. In order for transceiver B to transmit measured value Treply to transceiver A, at least two additional signals are needed including a data transmission signal for transmitting measured value Treply and a reply signal from transceiver A with respect to the data transmission signal.
(c) Transceiver A takes measured elapsed time value Tround from the end point of the transmitted signal A to the end point of the received signal B and value Treply received from transceiver B through the separate signal to calculate ToA=(Tround−Treply)/2. A value of a spatial distance between transceiver A and transceiver B can be calculated by multiplying ToA by signal transmission speed c of a medium.
By using the RT-ToA method, the location of a target device can be determined.
For example, in order to determine the position of the target device, a target device positioning system engaging three Access Points (APs) may use trilateration method in which the three APs respectively measure spatial distances d0, d1 and d2 to the target device through the RT-ToA method and form three circles having radiuses of d0, d1 and d2 centered about the positions of the APs for acquiring the intersecting point of the three circles.
A ranging and positioning method based on the RT-ToA has an advantage of a simple structure since no clock synchronization is needed between devices. Further, an analysis of the positioning precision for the target device reveals no significant difference between cases where the target device is located inside and outside of a cell formed of APs. The RT-ToA based method is characterized in that the margin of a positioning error stably amounts to a ranging error.
However, as described above, the RT-ToA method requires four signal exchanges between the transceivers. Accordingly, when the position of the target device is measured by using three APs, four signal exchanges are required between each AP and the target device to a total of twelve signal exchanges, thereby increasing loads on an air interface. Further, when there is a deviation between clock frequencies of the transceivers, the resultant ranging error becomes significant.
Meanwhile, an operational principle of the TDoA method is as follows:
(a) The target device transmits a blink signal.
(b) Each of three or more APs measures an arrival time of received blink signal and transmits the measured arrival time to a locating server.
(c) The locating server calculates a time difference by using the measured arrival time transmitted from each of the APs.
(d) The position of the target device is determined by the trilateration method of forming paraboloids with given position values of the APs and acquiring the common intersection point where the paraboloids meet each other.
The positioning system using the TDoA measures arrival times of signals received by the APs by simply having the target device transmit a blink signal once and the APs just receive the same signal.
The TDoA method has an advantage of a simple circuit configuration and a simple signaling method since the target device has only to transmit a signal and the APs have only to receive the same. However, the TDoA method is disadvantageous as for requiring additional system configuration and circuitry dedicated to the inherent clock synchronization between the APs.
Further, when analyzing the positioning precision for the target device with slight errors occurred in the arrival times of the signals received by the APs, the margin of the positioning error stably amounts to the error of the arrival time measurement if the target device is located inside the cell formed by the APs. However, if the target device is located outside the cell, the positioning error tends to become larger by a factor of several times to several dozen times of the arrival time measurement error depending on the position of the target device and thus is very unreliable.